The Sudden Motion Sensor (SMS) built into 2005 and later Mac notebooks can protect the hard drive from sudden and catastrophic events, such as the laptop slipping through your fingers and crashing to the ground.

The SMS system in a portable Mac is turned on by default. That’s a good thing for most Mac users, but it may present a problem for some of you, depending on how you use your Mac.

The SMS system is very helpful for hard drives, but not so much for Macs equipped only with an SSD, because SSDs don’t have moving parts that can be damaged by a sudden impact. So, having your Mac detect an impending impact and shut down the drive is mostly a waste of time. In some cases, it can have an effect on performance, causing stalls and spinning beach balls while your Mac suspends writes and reads from the drive as it waits on the SMS to say it’s OK to proceed.